Persons who work or exercise near motor vehicle traffic can be made safer by wearing clothing that highlights the person's presence to passing motor vehicles. To promote the safety of roadway workers and pedestrians, clothing manufacturers commonly produce bright clothing to make the wearer more conspicuous. Manufacturers also regularly secure retroreflective articles to the outer surface of the clothing to improve wearer conspicuity. Retroreflective articles are passive devices that return incident light back toward the light source. The articles highlight a person's presence to motorists at nighttime by reflecting light from the motor vehicle's headlamps back to the motor vehicle driver. The bright image displayed by the retroreflective article ultimately gives motorists more time to react.
Sometimes the retroreflective articles are colored for aesthetic reasons or to provide enhanced contrast for better daytime visibility. Frequently, fluorescent colors are used in conjunction with retroreflective sheeting to make the sheeting more conspicuous under daytime viewing conditions (see, for example, U.S. patent application Ser. No. 08/587,339 or corresponding International Publication WO 95/31739 and U.S. Pat. Nos. 3,830,682, 5,387,458, and 5,695,853).
Because retroreflective articles are regularly used on clothing, they must be able to withstand laundering conditions - otherwise, the articles cannot continue to serve their safety function after repeated washings. Investigators at the 3M Company who design retroreflective articles for use on clothing are aware of this problem, and they have developed launderably-durable retroreflective articles so that persons who wear retroreflective clothing remain conspicuously visible after their clothing has been laundered many times. U.S. Pat. Nos. 5,200,262, 5,283,101, 5,474,827, 5,645,938, 5,738,746, and 5,812,317 disclose examples of launderably durable retroreflective articles developed at 3M. These products typically comprise optical elements that are partially embedded in a specially formulated binder layer.
Investigators also recognize that the need to develop launderably durable retroreflective articles is particularly pronounced for clothing that regularly is worn in harsh environments. Examples of such clothing include firemen's jackets and construction workers' safety vests (see, for example, U.S. Pat. No. 4,533,592 to Bingham). These garments tend to get very dirty, very often, and therefore they are frequently cleaned under industrial laundering conditions. Industrial laundering conditions involve wash temperatures as high as 40 to 90.degree. C. (105 to 190.degree. F.) and pH values of 10 to 13. Some of the launderably durable retroreflective articles disclosed in the 3M patents mentioned above are capable of withstanding the more stringent industrial wash conditions.
In some retroreflective articles, a colored appearance has been achieved by placing a colored polymeric layer on top of the optical elements. Retroreflective articles that contain optical elements partially embedded in a polymeric top layer (also referred to as a cover film) are commonly referred to as "enclosed lens" retroreflective articles. In addition to providing color, the polymeric top film allows the article to be easily wiped clean, and the articles generally exhibit good retroreflective when wet. Examples of patents that disclose colored top films include U.S. Pat. Nos. 5,069,964 and 5,378,520. In these retroreflective articles, a dye or pigment is added to the top film. Commercially available products that have a colored top film include 3M Scotchlite.TM. 7960 and 7987 brand products.
An alternative to enclosed lens retroreflective articles are "exposed lens" retroreflective articles, which have the optical elements exposed to the ambient environment--that is, the optical elements are not covered by a polymeric top film. These articles generally include an exposed layer of transparent microspheres, a polymeric binder layer, and a reflective layer. The transparent microspheres are partially embedded in the binder layer and are partially exposed to the atmosphere, and the reflective layer is generally disposed between the microspheres and the binder layer.
Another kind of retroreflective article is an "encapsulated lens" retroreflective article. These articles are similar to enclosed lens articles in that they employ a top film over the layer of microspheres. Encapsulated lens retroreflective articles, however, differ from enclosed lens articles by having the top film encapsulate a pocket of air above the layer of microspheres. U.S. Pat. No. 4,025,159 to McGrath, U.S. Pat. No. 4,896,943 to Tolliver et al., U.S. Pat. No. 4,897,136 to Bailey et al., and U.S. Pat. No. 5,069,964 to Tolliver et al. disclose examples of encapsulated lens type products. In one variation of an encapsulated lens retroreflective sheeting (disclosed by Tung et al. in U.S. Pat. No. 4,678,695), transparent microspheres are partially embedded in a binder layer, and a clear or colored top film is disposed over the microspheres. The binder layer may be impregnated with a white pigment, or, alternatively, with a colored pigment to make a sheeting that displays a corresponding daytime color and exhibits nighttime reflection.
These three systems, exposed lens, enclosed lens, and encapsulated lens sheetings, have various advantages and disadvantages relative to one another, and coloring techniques applicable to one system are not necessarily applicable to the other. Exposed lens articles tend to be more flexible and simpler in construction but cannot be colored simply by including a dye in a top film because the articles have no top film. Enclosed lens and encapsulated lens articles, while being somewhat easier to color, generally suffer from the drawback of not being very useful at high temperatures because the polymeric top film can melt. Enclosed lens and encapsulated lens articles, therefore, do not rate as high as exposed lens articles when considering candidates for use on firefighters' jackets.
A variety of methods, however, have been employed to impart color to exposed lens retroreflective articles. In U.S. Pat. No. 3,700,305, for example, Bingham discloses an exposed lens retroreflective article that has alternating layers of different refractive index dielectric materials coated on glass microspheres. A colored layer, such as a fluorescent layer, is applied behind the dielectric reflector. Because the dielectric reflector is essentially transparent under daytime viewing conditions, the fluorescent layer imparts a daytime fluorescent color to the article. Under nighttime or retroreflective viewing conditions, however, the article is basically incapable of displaying the color of the underlying colored layer because incident light never strikes that layer: it is first reflected by the dielectric reflector back towards the light source. The patent is silent regarding durability under home wash or industrial laundering conditions.
Other methods of coloring an exposed lens retroreflective article are discussed briefly in U.S. Pat. Nos. 3,758,192, 4,102,562, and 5,200,262. In U.S. Pat. No. 3,758,192, Bingham discloses an exposed lens retroreflective article that has transparent microspheres partially embedded in a binder layer that contains flakes of nacreous (pearlescent) pigment and other various pigments or dyes. While this product can display a colored retroreflective image, there is nothing in the patent which shows that the product would be industrial wash durable. In U.S. Pat. No. 4,102,562 to Harper et al., an exposed lens retroreflective article is disclosed that can display a colored imagewise pattern. The article has transparent microspheres coated with a transparent dielectric mirror prepared as described in U.S. Pat. No. 3,700,305 to Bingham An ink layer that contains a pigment and a melamine is applied behind the reflective layer (see Example 2). Harper et al. state that the melamine reacts with the epoxide moiety of the adhesion promoting silane (see Example 2). Because the ink layer is disposed behind the reflective layer, the article, while being able to display the colored image under daytime viewing conditions, is not capable of displaying a colored retroreflective image. The patent also does not show that the retroreflective articles would be durable under industrial wash conditions. Wu-Shyong Li, in U.S. Pat. No. 5,200,262, partially embeds transparent microspheres in a binder layer that may be colored by a pigment or dye, preferably a black dye such as a chromium-azo dye. Li suggests the use of a metal layer or dielectric material as a reflector. The reflector is located on the embedded portion of the transparent microspheres. When a metal reflective layer is used, the color of the underlying binder layer is not noticeable under daytime or nighttime viewing conditions. And when a dielectric reflector is used, the color of the underlying binder layer is not noticeable under nighttime (i.e., retroreflective) viewing conditions. Li's product is, however, designed to withstand industrial wash conditions.
Ulf Olsen, in U.S. Pat. Nos. 5,344,705, 5,503,906, and 5,620,613, discloses exposed lens retroreflective articles that have a color layer printed on the embedded portion of a layer of transparent microspheres. The color layer typically contains a transparent pigment or dye that is substantially uniformly dispersed in a transparent resin. The color layer is disposed between the microspheres and a reflective layer, which reflective layer comprises reflective flakes in a transparent resin. Olsen also discloses that the color layer and the reflective layer may be replaced by a colored reflective layer comprising both colorant and reflective flakes in a transparent resin. While this product can display a colored image under retroreflective conditions, it does not indicate that good wash durability can be achieved under industrial wash conditions.
In U.S. Pat. Nos. 5,510,178, 5,612,119, 5,679,198, and 5,785,790, Ulf Olsen describes an exposed lens retroreflective product that has an imagewise colored coating disposed behind a transparent dielectric mirror that is coated on the backside of microspheres partially embedded in a binder layer. The colored image in this product is, however, not noticeable under retroreflective conditions; it can only be seen under daytime lighting conditions.